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The same frequency, but not connected via AC. There are multiple DC and Variable Frequency Transformer ties between the various interconnections.

https://en.wikipedia.org/wiki/North_American_power_transmiss...

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Exactly, DC is used for those links that would otherwise be out of synchronization. In Canada one of the DC links goes from the North American grid on the British Columbia Lower Mainland in Delta via a single underwater cable over to Vancouver Island. The water is the other conductor, and also keeps the cable cool.

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